The present invention relates generally to navigation systems, and more specifically the invention pertains to a means for underwater users to dynamically determine their position in a geodetically based coordinate system.
The need for accuracy, precision, and data registration in underwater positioning and navigation should be viewed as no less stringent than that existing on the surface or in the aerospace environment. The present invention includes a system which can be used to provide navigation service to free-ranging underwater users. Using this system an unlimited number of users can independently locate themselves and navigate. Underwater extensions of LORAN, OMEGA, or other radio-based navigation systems are also amenable to the technique.
The Global Positioning System and its antecedents cover the world's land and sea surfaces and the airspace volume, but currently do not support any of the sea volume. Covering some 71 percent of the earth, with an average depth of 3800 meters, the seas represent a volume of 1370 million cubic kilometers, presently inaccessible to conventional navigation services. Even if interest were limited to economic zones bounded by the continental shelf, five million new cubic kilometers of sea space could be made accessible with an appropriately-extended navigation service.
The marine technology community (e.g., people or groups conducting underwater scientific or defense research, or conducting commerce such as mapping or resource exploitation), uses various methods of underwater positioning and navigation. One determines the position of a surface support vehicle, deploys bottom-tethered beacons and relates their positions to that of the surface vehicle, then has users subsequently relate their position to those of the beacons. Another system determines the position of a surface support vehicle and subsequently relates the user's position to it in angle and range. A third relates data from one experiment to another based on the overlap of data and subsequently determined the relationship of some element or elements of this data to know positions.
It is noteworthy that, rather than locating itself under these scenarios, the user is most often "found" by the support vehicle Alternately, it "locates" itself relative to fixed, local monuments. Truly free-ranging users typically rely on inertial navigation systems, calibrated at the start of their mission. In any event, none of these users can make a direct determination of his dynamic position tied directly to a common, fixed, geodetically-based time-space coordinate system.
The Navstar Global Positioning System (GPS) is a space-based navigation system designed to allow an unlimited number of users to passively receive precise position, velocity, and time information anywhere on or above the earth's surface. Once the entire satellite constellation is in orbit, GPS will provide accurate, continuous positioning service for military and civilian ships, boats, aircraft, and land vehicles. GPS is expected to become a primary navigation aid during the 1990s.
Articles in the professional literature describing the proposed or actual application of the Global Positioning System and other radio-based navigation services to user vehicles concentrate on the sea surface, terrestrial, and aerospace environments. Vehicles in these environments may use GPS as primary means of dynamic position determination or may use GPS to calibrate an inertial navigation system (INS). Underwater GPS users must rely on their inertial navigation system for primary dynamic position determination and must surface to access the GPS signal and calibrate their INS. Furthermore, none of the current methods of underwater positioning and navigation provide for direct determination of the user's dynamic position in a global reference system.
The task of extending the use of the Global Positioning System into a passive underwater navigation system is alleviated, to some extent, by the systems described in the following U.S. Pat. Nos., the disclosures of which are specifically incorporated by reference:
U.S. Pat. No. 4,924,448 issued to Gaer; PA0 U.S. Pat. No. 4,894,662 issued to Counselman; PA0 U.S. Pat. No. 4,751,512 issued to Longaker; PA0 U.S. Pat. No. 4,639,903 issued to Redolfi; PA0 U.S. Pat. No. 4,622,557 issued to Westerfield; and PA0 U.S. Pat. No. 4,445,118 issued to Taylor et al.
The patents identified above relate to various systems for mapping and navigation. In particular, the Gaer patent describes a system and method for ocean bottom mapping and surveying in which two ships travel on parallel courses, but some distance apart. Each ship transmits a fan shaped sonic pulse downward abut the vertical axis, and transverse to the ship's longitudinal axis. The backscattered echo is processed for depth. The forward scattered portion of this pulse is received by the other ship, and is analyzed to determine ocean bottom depths of the swath between the ships. Precise global navigation fixes and relative intership positional fixes are provided by the GPS.
The Counselman patent relates to a method and apparatus for determining a ship's position by using signals transmitted by GPS satellites. A precise measurement of the ship's range to each satellite is made based upon the L1 center frequency carrier phase. A correction for ionospheric effects is determined by simultaneous observation of the group delays of the wide bandwidth P code modulations in both the L1 and L2 bands. The group delays are determined by measuring the phases of carrier waves implicit in the spread-spectrum signals received in both bands. The unknown biases in the L1 center frequency carrier phase range measurements are determined from simultaneous, pseudorange measurements with time averaging. The instantaneous position of the ship may then be determined from the ranges so determined.
The Longaker patent describes a differential navigation system. A reference receiver of known location tracks a navigation information service, computes differential data with respect to that information, and communicates the data to a transmitting unit. The transmitting unit transmits differential data via a commercial geosynchronous earth satellite relay to a mobile user. The mobile user receives the relayed signal with a non-directional, circularly polarized, non-stabilized antenna.
The Redolfi patent relates to a high fidelity sound delivering system which can be mounted on a floating buoy to deliver sound both underwater and above. The system comprises a hollow sphere which is partially submerged and almost filled with water. A plurality of air-filled balls keep the sphere afloat. A dynamic sound transducer inside the sphere has its diaphragm bolted to the wall of the sphere. A second piezoelectric high frequency transducer is banded to the inside surface of the sphere. The mechanical vibration of the transducers are imparted to the walls of the sphere, which in turn generate sound waves directed both under and above the water.
The Westerfield patent describes a transdigitizer for relaying signals from GPS satellites. An RF stage, which comprises an antenna, filter and preamplifier receiver, filters and amplifies the signals. A converter stage, with bandpass filter, converts the GPS signal to a lower frequency. A final downconverter converts the signal to a base band frequency, and a zero crossing detector amplifies and quantizes the signal. A local oscillator controls a frequency synthesizer to latch the signal from the zero crossing detector in a flip flop, which in turn is used to control a quadraphase modulator. Signals from the modulator are then amplified and transmitted by an antenna. The transdigitizer thus transmits the translated GPS signal plus data gathered on board its host platform.
The, Taylor et al patent relates to a navigation system in which the NAVSTAR/GPS system constantly broadcasts a spread spectrum, pseudorandom code which is unique for each of the satellites in the GPS system. The nature of such a system allows for identification of the signal source. These signals are transmitted on a downlink to the user terminals. A transmitter accepts information from the GPS master control station, and in turn transmits an additional frequency shift keyed (FSK) signal on a reference channel to the terminals, which may then determine the position or velocity of the terminal. This transmitted signal contains GPS satellite position data as to which GPS satellites are in best view, Doppler prediction information, and a PRN code generator signal. It is this additional signal that allows for greater relaxation of local oscillator and signal processing requirements for the terminal receivers.
Although these patents relate to navigation systems which utilize the GPS system, they do not describe a system by which underwater users could dynamically determine their postion in a geodetically based coordinate system.